With all due respect this is the most ridiculous thing imaginable .. The Dream is about 200 feet longer and 20 feet wider, if that is too much extra far to walk you either have to be disabled or incredibly out of shape. Dont know about you, but any normal human could walk this extra distance in about 2 minutes flat at a moderate pace.
To be real, there is a lot more to do on the Dream (mini golf, Midship Detective Agency, Aquaduck, etc), the main restaurants are bigger and more heavily themed like you would expect from Disney, and yes of course there are more people to go along with the larger spaces. There is a reason why the Dream is more expensive.
With that being said I dont think you can wrong either way for your first cruise.